Conventionally, a pseudorandom signal has been used as a code as a key, or used as a diffusion signal of a CDMA (Code Division Multiple Access) method. The following methods are conventional methods for creating a pseudorandom signal.
The first method is a method of creating and using various pseudorandom sequences (binary signal rows). Conventionally, there have been several types of pseudorandom sequences: an M-sequence (a maximum period sequence), a gold sequence, a barker sequence, and various sequences deriving from the M-sequence. Signal amplitudes thereof are binary (0 or 1), and phases are random. Level-converting these sequences (the binary signal rows) into appropriate values and using as signals makes pseudorandom signals.
These pseudorandom signals have different phases when generating polynomials as sources of generation differ, and thus become different signals. In addition, a cross correlation of different sequences exhibits a property different from a property of an autocorrelation of the sequence itself, and two signals can be distinguished, whether identical or different, by using this property. Additionally, since these pseudorandom signals are formed from generating polynomials, these pseudorandom signals become periodic signals though random. It is noted that these pseudorandom signals become signals random in one period. Due to these properties, these pseudorandom signals are suitable for diffusion signals used in the CDMA method.
The second method uses a noise in the natural world. In the natural world exists a signal, such as a thermal noise, which is likely to be usable as a random signal. Although this includes many types, each of these types is random, and thus is not reproducible. From these reasons, noises in the natural world are rarely used as diffusion signals used in the CDMA method. Besides, as a signal used for system identification, a signal having a normality and a spectral property within tolerable ranges is selected, and is used as the signal for system identification.
The pseudorandom signals generated by the conventional first method are limited in numbers. The number of the pseudorandom signals is theoretically determined according to types thereof. In the CDMA communication method, the number of the pseudorandom signals used as the diffusion signals equals the number of channels; therefore, the limitation on the number of the pseudorandom signals has caused a problem of limiting an increase in the number of the channels.
In addition, since commerce on the Internet began to attract attention, code as a key for the Internet commerce has attracted attention, and it has been required to further enhance security. For enhancing this security, it is important to increase complexity of the code itself, and at the same time, to increase the number of the codes. In general, a difficulty of code decryption is defined as a probability of finding out a code as a key, when using any temporal calculation method, i.e., a method of attempting to find the code as the key by using a computer. The smaller this probability is, the harder the code becomes to decrypt. That is because, when the number of the codes themselves is large, it takes a longer time to find out the code so that the decryption becomes difficult. However, since the pseudorandom signals generated by the conventional first method are limited in numbers, there has been a problem that there is a limit in enhancing the security of the code.